Production of integrated circuits, microelectronic devices, and micro-electro mechanical devices, (MEM's) involve multiple processing steps many of which incorporate water as either a carrier of chemistry, or a media to facilitate the removal of process byproducts. The evolution of materials and processes has been lead by a drive toward smaller feature sizes and more complex microdevices. In some cases, the use of water in these evolving processes has resulted in challenges whereby deleterious effects of water and byproducts carried by water have been seen. The unique physical properties of dense carbon dioxide in a liquid or supercritical state are of particular interest in preventing certain of these pitfalls.
One such process where dense CO2 is of practical application relates to prevention of surface tension or capillary force induced image collapse. This is of particular interest during the aqueous development of micro-lithographic images using photoresists. Photoresists are photosensitive films used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed, through a photomask or by other techniques, to a source of activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask (or other pattern generator) to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate. See. e.g., U.S. Pat. No. 6,042,997.
A photoresist can be either positive-acting or negative-acting. For negative acting resists, the solubility of the exposed region is decreased such that it remains on the wafer during development while the non-exposed region is removed. For positive acting resists the solubility of the exposed region increases in the developer solution, so it is removed during the development step leaving the unexposed region unaffected. Positive and negative acting resist materials typically incorporate chemical functionality that undergoes a transformation upon exposure to UV light at a given wavelength. The transformation is often referred to as a “polarity switch” because polymer polarity increases or decreases are often the driving force for changes in the solubility of the polymer in the developing solution. This transformation is facilitated by the incorporation of photoacid generators (PAG's) or photobase generators (PGB's) into the resist compositions. The acid and base moieties are typically generated upon exposure to the appropriate source of radiation followed by heat. The developer solutions are typically aqueous, and are typically dried from the substrate before further processing.
Capillary forces present in the aqueous drying of imaged resist patterns can result in resist deformation and pattern collapse. This problem becomes particularly serious as lithography techniques move toward smaller image nodes with larger aspect ratios. Researchers have suggested that collapse problems associated with aqueous drying will affect the 130-nm technology node, and will become more prevalent in subsequent technologies as aspect ratios increase.
Researchers at both IBM and NTT have suggested that the use of carbon dioxide in supercritical resist drying (SRD) may reduce image collapse and film damage. See, e.g., H. Namatsu, J. Vac. Sci. Technol. B 18(6), 3308-3312 (2000); D. Goldfarb et al., J. Vac. Sci. Technol B. 18(6) 3313-3317 (2000). However, while the absence of surface tension and the accessible critical temperature and pressure of CO2 have been touted as positives factors for this drying approach, the relatively low solubility of water in the supercritical phase has also been described as a challenge that may necessitate the use of chemical adjuncts to increase the transport capacity of the fluid. Researchers at IBM and NTT have demonstrated the use of certain surfactants in supercritical fluid-aided drying. However, the surfactant is described as being incorporated into a hexane pre-rinse in “indirect SRD” See, e.g., Goldfarb et al., supra, or only particular surfactants have been incorporated into the carbon dioxide in “direct SRD”. In both the direct and indirect drying methods the choice of surfactants and co-solvents is limited by what is described as compatibility issues leading to resist damage. Accordingly, there remains a need for new approaches to SRD using carbon dioxide.
Another problem with drying of surfaces on microelectronic substrates (e.g. photoresist coated semiconductor wafers, MEMS, opto-electronic devices, photonic devices, flat panel displays, etc) is the complete removal of aqueous processing, cleaning or rinsing solutions without leaving a residue, commonly referred to as a drying watermark. These watermarks result from the concentration of solutes in the aqueous processing, cleaning, or drying fluid, as said fluid is dried. In many microelectronic, optical, micro-optical, or MEMS structures this watermark can negatively impact the manufacturing yield or ultimate performance of the device. There needs to be an effective method to remove (clean) water-based fluids from surfaces that eliminates the concentration and ultimate deposition of entrained solutes—eliminating watermarks.
One such challenge comes in the manufacturing of MEM's devices. Wet-processing steps generally culminate with a rinse and dry step. Evaporative drying causes water with low levels of solutes that is pooled on the surface and in various micro-features to concentrate in locations that maximize the surface area of the pool. As a result, these drying steps can lead to the concentration of once dissolved solutes in close proximity to or on motive parts. The deposited materials which can be organic or inorganic in nature contribute to stiction, the locking of the motive part such that it cannot be actuated. “Release stiction” as it is termed during the manufacturing step results, is believed to be derived from adhesive and Van der Waals forces and friction. The forces generated by this phenomenon can completely incapacitate motive parts on MEM's devices.
To combat stiction manufacturers of MEM's devices use solvents such as small chain alcohols that reduce surface tension during the rinse step and facilitate a more even drying process. However, these steps alone have not eliminated the occurrence of stiction. Supercritical CO2 has been proposed for drying microstructures, (see Gregory T. Mulhern “Supercritical Carbon Dioxide Drying of Micro Structures”) where surface tension forces can cause damage. Researchers at Texas Instruments Inc. among others (see, e.g., U.S. Pat. No. 6,024,801) have demonstrated that supercritical CO2 can be used to clean organic and inorganic contaminants from MEM's devices prior to a pacification step, thus limiting stiction.
These technologies utilizing supercritical CO2 do not limit stiction by combination of drying and cleaning where water and solutes are removed simultaneously so to avoid the concentration of water and solutes at specific site. Technologies are needed that can prevent release stiction through an integrated process of drying, cleaning, and surface pacification.
Other examples of drying and cleaning challenges related to aqueous wet-processing steps come in the formation of deep vias for interlayer metalization in the production of integrated circuits. These vias, formed by methods known to those familiar with the art, typically have large critical aspect ratios creating geometries that can be difficult to clean residues from. Furthermore, wet-processing steps and rinses with traditional fluids such as water leave once dissolved solutes behind upon evaporative drying. These solutes deposited at the bottom of the vias can inhibit conduction upon metalization lowering functional yields.
Technologies are needed that remove water (dry) and dissolved solutes (clean) from vias after wet processing steps, thus reducing yield losses.
Additionally, the emergence of porous dielectric materials used in manufacturing integrated circuits will require that a cleaning media be able to ideally wet smaller and smaller feature size to remove device damaging contaminants and process byproducts without damaging structure or materials. Dense fluid CO2 is desirable because of low or no surface tension and very low viscosity. However, chemical adjuncts such as co-solvents, surfactants, oxidants, etchants, and stabilizers, must be effectively removed after they serve the cleaning need. This can be very challenging in a fluid with excellent transport properties and variable solvent properties as a function of temperature and pressure. This fluctuation of solvency can result in the unwanted deposition of materials into and on these porous substrates without effective rinsing. What is needed is a process that allows for the efficient and effective removal of not only contaminants and process byproducts but also chemical reagents.